MEMS micromirror and micromirror array

ABSTRACT

A micro-electro-mechanical-system (MEMS) micromirror array has an array of micromirrors on a support structure. Each micromirror is pivotally attached to the support structure by a resilient structure. The resilient structure defines a pivot axis. There is an array of electrostatic actuators for pivotally driving the array of micromirrors about the pivot axis. Each electrostatic actuator comprises a first part carried by the support structure, and a second part carried by the corresponding micromirror. An electrostatic sink is mounted to the support structure that shields at least one micromirror from spurious electrostatic actuation.

TECHNICAL FIELD

This relates to a MEMS (Micro-Electro-Mechanical System) micromirrorused for one or two dimensional tilting micromirror arrays with a highfill factor.

BACKGROUND

The MEMS (Micro-Electro-Mechanical System) mirrors and mirror arrayshave wide applications in the light process and fiber optic networkssuch as in optical cross-connect switches, attenuators, wavelengthblocker, dynamic gain equalizer, configurable grating and tunable filteretc. The MEMS mirror arrays with high fill factors and one or two axesrotation have particular importance in the wavelength divisionmultiplexing systems. The fill factor is generally defined as the ratioof the active area to the total area in an array. The high fill factorimproves the shape of the optical channel and reduces the optical lossin the system. A micromirror with two axes of rotation can provideswitching of the optical beam among the channels while avoidingundesirable optical transient cross-talk during switching, and achievingvariable optical attenuations.

There are a number of actuation methods for the MEMS micromirror arraysuch as electromagnetic actuation disclosed in U.S. Pat. No. 6,760,145(Taylor et al.) entitled “Actuator for dual-axis rotation micromirror”,thermal actuation disclosed in U.S. Pat. No. 7,091,057 (Gan et al.)entitled “Method of making a single-crystal-silicon 3D micromirror”, andelectrostatic actuation disclosed in U.S. Pat. No. 7,095,546 (Mala etal.) entitled “Micro-electro-mechanical-system two dimensional mirrorwith articulated suspension structures for high fill factor arrays”.Electrostatic actuation is favored due to its low power consumption andrelative simple structure and small footprint.

Existing micromirrors with electrostatic actuation fall into twocategories: vertical combdrive type micromirrors and parallel plate typemicromirrors. The drawback for conventional vertical combdrive typemicromirrors is that is generally fails to form the high fill factorarrays due to its typical gimbaled and framed structure. Since it isdifficult to reduce the gap between adjacent micromirrors, it is hard toform a mirror array with high fill factor. An example of this type ofMEMS micromirrors is found in U.S. Pat. No. 6,822,776 (Hah et al.)entitled “Scanning micromirror for optical communication systems andmethod of manufacturing the same”.

It is much easier to form high fill factor minor arrays based on theparallel plate type of electrostatic actuators. The majority of existinghigh fill factor micromirror array designs use parallel plate type ofelectrostatic actuators, such as those taught in U.S. Pat. No. 7,095,546(Mala et al.) entitled “Micro-electro-mechanical-system two dimensionalmirror with articulated suspension structures for high fill factorarrays”, U.S. Pat. No. 6,934,439 (Mala et al.) entitled “Plano MEMSmicromirror”, U.S. Pat. No. 6,694,073 (Golub et al.) entitled“Reconfigurable free space wavelength cross connect”, U.S. Pat. No.6,781,744 (Aksyuk) entitled “Amplification of MEMS motion”, U.S. Pat.No. 6,778,728 (Taylor et al.) entitled “Micro-electro-mechanical mirrordevices having a high linear mirror fill factor”, U.S. Pat. No.7,209,274 (Van Drieenhuizen et al.) entitled “High fill-factor bulksilicon mirrors” and U.S. Pat. No. 7,053,981 (Bleeker) entitled“Lithographic apparatus and device manufacturing method”. The advantageof using a parallel plate electrostatic actuator is that no typicalgimbaled structure or frame is required for the design. As such, the gapbetween the mirrors can be very small to form a high fill factor mirrorarray.

SUMMARY

According to an aspect, there is provided amicro-electro-mechanical-system (MEMS) micromirror array, comprising anarray of micromirrors on a support structure, each micromirror beingpivotally attached to the support structure by a resilient structure,the resilient structure defining a pivot axis. The MEMS micromirrorarray further comprises an array of electrostatic actuators forpivotally driving the array of micromirrors about the pivot axis, eachelectrostatic actuator comprising a first part carried by the supportstructure, and a second part carried by the corresponding micromirror.The MEMS micromirror array further comprises an electrostatic sinkmounted to the support structure that shields at least one micromirrorfrom spurious electrostatic actuation.

According to another aspect, the electrostatic sink may be one or moreof an electrical ground, a physical barrier, an electrode on the supportstructure. The electrostatic sink may shield at least one micromirrorfrom spurious actuation by an adjacent electrostatic actuator and mayshield at least one micromirror from accumulated electrostatic charge onthe support structure.

According to another aspect, the electrostatic actuator may be avertical comb drive. One of the first part of the vertical comb drive orthe second part of the vertical comb drive may comprise fingers that areenclosed within an outer perimeter of the other of the first part or thesecond part. The fingers may be carried by a carrier portion that isperpendicular to the pivot axis, the carrier portion being connected toan external portion that is outside the outer perimeter of the other ofthe first part and the second part. The fingers may be parallel to thepivot axis. The fingers may be angled relative to the pivot axis.

According to another aspect, each micromirror may be symmetrical aboutthe pivot axis. The micromirrors in the array may be staggeredperpendicularly to the pivot axis.

According to another aspect, there may be a cavity between themicromirrors and the support structure, and a physical barrier mayseparate adjacent cavities to prevent pneumatic actuation of adjacentmicromirrors due to movement of an adjacent micromirror. The physicalbarrier may comprise an intermediate support structure.

According to another aspect, the micromirrors may be formed from a firstlayer of material, the electrostatic sink may be formed from a secondlayer of material, and the support structure may be formed from a thirdlayer of material.

According to another aspect, at least one micromirror may comprise asecond electrostatic actuator for pivoting the micromirror about asecond pivot axis.

According to another aspect, the resilient structure may comprise afirst portion having an I beam connected to a composite structure, and asecond portion that is symmetrical to the first portion, the first andsecond portions defining the pivot axis. The composite structure may beone of one or more dual I beam structures, one or more V-shapedstructures, and combinations thereof.

According to another aspect, there is provided a MEMS micromirrorstructure, comprising a micromirror mounted on a support structure by aresilient structure, and an electrostatic actuator for moving themicromirror relative to the support structure. The electrostaticactuator comprises a first part carried by the support structure, and asecond part carried by the corresponding micromirror. A latch is mountedto the support structure by a movable portion that moves in response toan applied voltage between a latching position and a release position asthe applied voltage is varied, wherein, in the latching position, thelatch secures the micromirror in a desired position. The movable portionmay be a thermal arched beam actuator. The movable portion may comprisefirst and second parallel thermal connectors that expand at differentrates in response to the applied voltage.

According to another aspect, there is provided a MEMS micromirrorstructure, comprising a micromirror mounted on a support structure by aresilient structure; and an electrostatic actuator for pivotally drivingthe micromirror. The electrostatic actuator comprises a first partcarried by the support structure, and a second part carried by thecorresponding micromirror. The resilient structure comprises a firstportion and a second portion that is symmetrical to the first portion.Each of the first portion and the second portion comprises an I beamconnected to a composite structure. The composite structure may be oneof one or more dual I beam structures, one or more V-shaped structures,and combinations thereof.

According to another aspect, there is provided a MEMS micromirror,comprising a micromirror mounted on a support structure by a resilientstructure. The resilient structure permits movement along an axis thatis perpendicular to the support structure and resists movement in anydirection perpendicular to the axis. The MEMS micromirror furthercomprises an electrostatic combdrive actuator that has a first partmounted on the support structure, and a second part mounted on themicromirror. The first part and the second part provide an actuatingforce to the micromirror to move the micromirror along the axis. Theelectrostatic combdrive actuator may comprise more than one first andsecond parts spaced evenly about an outer perimeter of the micromirror.The resilient structure may comprise more than one resilient structuresspaced evenly about an outer perimeter of the micromirror. The supportstructure may be formed from a first layer of material, the first partof the electrostatic combdrive may be formed from a second layer ofmaterial, and the micromirror, the second part of the electrostaticcombdrive and the resilient structure may be formed from a third layerof material.

According to another aspect, there is provided a singlemicro-electro-mechanical-system (MEMS) micromirror and a MEMSmicromirror used in high fill factor mirror arrays includes at least onemoveable mirror, flexible hinges to connect the mirror to the fixedanchors, a wall structure surrounding the mirror, and supportingmaterial to support the wall structure and anchors. The wall structureseliminate the electrical and mechanical crosstalk between any adjacentmirrors in the mirror array. The whole wall structure or parts of thewall structure are used as electrostatic actuation components. The wallstructure is also working with actuation electrodes on the supportmaterial to form electrode gap with high aspect ration to reduce oreliminate the mirror tilting drifting caused by the charged dielectricmaterials within the gap.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features will become more apparent from the followingdescription in which reference is made to the appended drawings, thedrawings are for the purpose of illustration only and are not intendedto be in any way limiting, wherein:

FIG. 1 a is a perspective view of a prior art micromirror array using aconventional parallel plate type electrostatic actuator.

FIG. 1 b is a detailed perspective view of Detail “A” in FIG. 1 a.

FIG. 2 a is a perspective view of a micromirror using wall structuresurrounding the mirror.

FIG. 2 b is a perspective view in section of a micromirror using wallstructure surrounding the mirror.

FIG. 2 c is a detailed view of a micromirror using wall structuresurrounding the mirror.

FIG. 3 a is a rear perspective view of high fill factor mirror arrayusing the mirror show in FIG. 2 a.

FIG. 3 b is a front perspective view of high fill factor mirror arrayusing the mirror show in FIG. 2.

FIG. 4 a is a perspective view of a single I beam hinge.

FIG. 4 b is a top plan view of the single I beam hinge.

FIG. 5 a is a perspective view of a double I beam hinge.

FIG. 5 b is a top plan view of the double I beam hinge.

FIG. 5 c is a perspective view of a hinge with a combination of a singleI beam and double I beam.

FIG. 5 d is a perspective view of a hinge with a combination of a singleI beam and two cascaded double I beams.

FIG. 5 e is a perspective view of a hinge with a combination of a singleI beam and three cascaded double I beams.

FIG. 5 f is a perspective view of a V shaped hinge.

FIG. 5 g is a perspective view of a hinge with a combination of a singleI beam and a V shaped hinge

FIG. 6 a is a perspective view of the micromirror device with atrenching and refilling region on the mirror connector.

FIG. 6 b is an enlarged perspective view of the trenching and refillingregion on the mirror connector shown in Detail “C” of FIG. 6 a.

FIG. 7 is a perspective view of pattern variations of the trenching andrefilling regions.

FIG. 8 is a perspective view of micromirror device with the wallstructure surrounding only one of the two mirrors.

FIG. 9 a is a perspective view of micromirror array device using themicromirror shown in FIG. 6.

FIG. 9 b is a perspective view of micromirror array device using themicromirror shown in FIG. 8.

FIG. 9 c is a perspective view of micromirror array device using themicromirror shown in FIG. 6.

FIG. 10 a is a perspective view of a micromirror array device using themicromirror shown in FIG. 6.

FIG. 10 b is a perspective view of micromirror array device using themicromirror shown in FIG. 8.

FIG. 11 is a perspective view of a micromirror device.

FIG. 12 is a side elevation view in section of the interference ofcharged dielectric material within the electrode gap.

FIG. 13 a is a side elevation view in section of the shielding effect ofan electrode gap with high aspect ratio.

FIG. 13 b is a side elevation view in section of the shielding effect ofan electrode gap variation with high aspect ratio.

FIG. 13 c is a side elevation view in section of the shielding effect ofan electrode gap variation with high aspect ratio.

FIG. 14 a is a side elevation view in section of the shielding effect ofan electrode gap with high aspect ratio and removed dielectric materialwithin the gap.

FIG. 14 b is a side elevation view in section of the shielding effect ofan electrode gap variation with high aspect ratio and removed dielectricmaterial within the gap.

FIG. 14 c is a side elevation view in section of the shielding effect ofan electrode gap variation with high aspect ratio and removed dielectricmaterial within the gap.

FIG. 15 a is a perspective view in section of the micromirror usingshielding effect of the electrode gap with high aspect ratio.

FIG. 15 b is a perspective view in section of the micromirror variationusing shielding effect of the electrode gap with high aspect ratio.

FIG. 15 c is a perspective view in section of a micromirror variationusing shielding effect of the electrode gap with high aspect ratio.

FIG. 16 a is a perspective view of a glass plate with electrodestructure using shielding effect of the electrode gap with high aspectratio.

FIG. 16 b is a perspective view of the glass plate with electrodestructure using shielding effect of the electrode gap with high aspectratio.

FIG. 16 c is a perspective view of the glass plate with electrodestructure using shielding effect of the electrode gap with high aspectratio.

FIG. 17 a is a perspective view in section of the micromirror using anelectrostatic vertical comb drive.

FIG. 17 b is a perspective view in section of the micromirror using anelectrostatic vertical comb drive and a trenching and refilling regionon its mirror connector.

FIG. 17 c is a perspective view in section of the micromirror using anelectrostatic vertical comb drive without the wall structure between twoadjacent mirrors.

FIG. 17 d is a perspective view in section of the micromirror using anelectrostatic vertical comb drive and a trenching and refilling regionon its mirror connector, and without the wall structure between twoadjacent mirrors.

FIG. 18 a is a perspective view in section of the micromirror using anelectrostatic vertical comb drive and an electrode structure using theshielding effect of the electrode gap with high aspect ratio.

FIG. 18 b is a perspective view in section of the micromirror variationusing an electrostatic vertical comb drive and an electrode structureand using the shielding effect of the electrode gap with high aspectratio.

FIG. 18 c is a perspective view in section of a micromirror variationusing an electrostatic vertical comb drive and an electrode structureand using the shielding effect of the electrode gap with high aspectratio.

FIG. 18 d is a perspective view in section of a micromirror variationusing an electrostatic vertical comb drive and an electrode structureand using the shielding effect of the electrode gap with high aspectratio, and without the wall structure between two adjacent mirrors.

FIG. 18 e is a perspective view in section of a micromirror using anelectrostatic vertical comb drive and an electrode structure and usingthe shielding effect of the electrode gap with high aspect ratio, andwithout the wall structure between two adjacent mirrors.

FIG. 18 f is a perspective view in section of a micromirror variationusing an electrostatic vertical comb drive and an electrode structureand using the shielding effect of the electrode gap with high aspectratio, and without the wall structure between two adjacent mirrors.

FIG. 19 a is a perspective view of a comb finger design.

FIG. 19 b is a perspective view of a comb finger design variation.

FIG. 19 c is a perspective view of a comb finger design variation.

FIG. 19 d is a perspective view of a comb finger design variation.

FIG. 20 a is a perspective view of a micromirror variation using anelectrostatic vertical comb drive to increase mirror tilting angle.

FIG. 20 b is a perspective view of a micromirror variation using anelectrostatic vertical comb drive to increase mirror tilting angle.

FIG. 21 a is a perspective view of a micromirror variation using atrenching and refilling region to reduce the mirror array die size.

FIG. 21 b is a perspective view of a micromirror variation using anelectrostatic vertical comb drive to reduce the mirror array die size.

FIG. 21 c is a perspective view of a micromirror variation usingelectrostatic vertical comb drive and extension beams.

FIG. 22 a is a perspective view of a micromirror array variation usingthe micromirror shown in FIG. 21 a.

FIG. 22 b is a perspective view of a micromirror array variation usingthe micromirror shown in FIG. 21 b.

FIG. 22 c is a perspective view of a micromirror array variation usingthe micromirror shown in FIG. 21 c.

FIG. 22 d is a perspective view of a simplified micromirror array usingthe micromirror shown in FIG. 22 c.

FIG. 23 a is a perspective view in section of a micromirror usingvertical comb drive for two axes tilting with a detailed view of Detail“D”.

FIG. 23 b is a perspective view in section of a micromirror variationusing a vertical comb drive for two axes tilting.

FIG. 24 a is a perspective view of a micromirror variation using themicromirror shown in FIG. 23 b.

FIG. 24 b is a perspective view of a micromirror variation using themicromirror shown in FIG. 23 a.

FIG. 25 is a perspective view of a micromirror variation using themicromirror shown in FIG. 23 a or FIG. 23 b.

FIGS. 26 a-26 k are side elevation views in section of major processsteps of the first fabrication method to fabricate a micromirror andmicromirror array.

FIGS. 27 a-27 k are side elevation views in section of major processsteps of the second fabrication method to fabricate a micromirror andmicromirror array.

FIG. 28 is a perspective view of a micromirror array device usingmicromirrors with thin and tall mechanical walls between adjacentmirrors to prevent the electrical and mechanical crosstalk.

FIG. 29 a is a perspective view of a single tilting micromirror deviceusing the actuation structures and fabrication method with a detailedview of Detail “E”.

FIG. 29 b is a perspective view of a single tilting micromirror deviceusing the actuation structures and fabrication method with twoelectrically isolated bottom electrodes under the titling mirror.

FIG. 29 c is a perspective view of a single tilting micromirror devicevariation using the actuation structures and fabrication method.

FIG. 29 d is a perspective view of a single micromirror device variationusing the actuation structures and fabrication method.

FIG. 30 a is a perspective view of a single tilting micromirror deviceusing the actuation structures and fabrication method with a micromirrortilting position latching structure and its thermal actuator afterfabrication.

FIG. 30 b is an enlarged perspective view of Detail “F” in FIG. 30 a ofa micromirror tilting position latching structure.

FIG. 31 a is a perspective view of a single tilting micromirror deviceusing the actuation structures and fabrication method with a micromirrortilting position latching structure and its thermal actuator afterthermal actuation.

FIG. 31 b is an enlarged perspective view of Detail “G” in FIG. 31 a ofa micromirror tilting position latching structure after thermalactuation.

FIG. 32 a is a perspective view of a single tilting micromirror deviceusing the actuation structures and fabrication method with a micromirrortilting position latching structure and its thermal actuator in thelatching state.

FIG. 32 b is an enlarged perspective view of Detail “H” in FIG. 32 a ofa micromirror tilting position latching structure in the latching state.

FIG. 33 a is a perspective view of a single tilting micromirror deviceusing the actuation structures and fabrication method with a micromirrortilting position latching structure and its thermal actuator in thelatching state, with trenching and refilling regions used to preventelectrical interference.

FIG. 33 b is an enlarged perspective view of Detail “I” in FIG. 33 a ofa micromirror tilting position latching structure in the latching state,with trenching and refilling regions used to prevent electricalinterference.

FIG. 34 is an enlarged perspective view of a thermal actuator variationfor latching the micromirror at its desired tilting position.

DETAILED DESCRIPTION

A micromirror with parallel plate type electrostatic actuators known inthe prior art is shown in FIG. 1. The mirror 1, 2 and 3 are coated withoptical reflective materials such as an optical reflective metal film,and is supported by hinges 7, 8 and 9 which are connected to theiranchors. The fixed actuation electrodes 4, 5 and 12 are located belowmirror 2, 3 and 1. Mirror 1, 2 and 3, hinges 7, 8 and 9 can be made ofheavily doped electrically conductive silicon for good electricalconductivity. When the actuation voltage is applied between mirror 2 andelectrode 4, the resulting electrostatic force between mirror 2 andelectrode 4 will pull the mirror 2 towards electrode 4 and cause thedeformation of hinges 8 and rotation of the mirror 4 around the hinges8. When the electrostatic force is balanced with the mechanicalrestoring force of the deformed hinges 8, the mirror 2 will stabilize ata tilting angle.

There are some major disadvantages of such mirror array structures. Onedisadvantage is the electrical interference. The electrical fieldbetween mirror 2 and electrode 4 will interfere with theposition/movement of adjacent mirror 1 and mirror 3. In other word, theelectrical field generated by any mirror and its electrode will affectthe position/movement of its adjacent mirrors. This electricalinterference cause adjacent mirror position control difficult andcomplex.

Another disadvantage is mechanical interference. When the actuationvoltage is applied between mirror 2 and electrode 4, the resultingelectrostatic force between mirror 2 and electrode 4 will pull themirror 2 towards electrode 4. The air between the movable mirror 2 andthe fixed electrode 4 during this fast mirror tilting movement will beeither compressed or decompressed. As such, the air flow will be formed.The air flow resulting from the switching of mirror 2 will thereforeinterfere with the adjacent mirror 1 and 3, and cause mirror 1 and 3 tochange their positions.

A third disadvantage is the drift in the mirror tilting angle due toelectrical charging in the dielectric materials. The electrodes 12, 4and 5 are on top of a dielectric material 6 to provide electricalisolation. In order to have a larger controllable titling angle for themirror, the gap between the fixed actuation electrode and the mirror hasto be increased. This increased gap results in a higher actuationvoltage, such as over 100 V, to obtain a couple of degrees of mirrortitling. This higher actuation voltage can cause electrical charging ofthe dielectric material 6 within the gap 11 between two electrodes 4 and5. The amount of charging varies with the time and the applied actuationvoltage as well as the packaging condition etc. The electrical fieldgenerated from these variable charges will in turn cause an undesireddrift in the tilting angle of the mirror 2 and 3.

The disadvantages discussed above may lead to spurious mirror movement,or in other words, unintended and “false” movement of a mirror that mayaffect the operation of a device. This may include movement where thereshould be none, or reduced movement or lack of movement when actuated.

To overcome these disadvantages and reduce the effect of spurious mirrormovement, a thin and electrically conductive wall structure around themirror may be utilized. One example of the mirror with such a thin wallstructure is shown in FIGS. 2 a-2 c. The mirror 25 with opticalreflective coating 19 is connected to the anchor 18 using hinge 26. Themirror 25, hinge 26 and anchor 18 can be made of electrically conductivematerial such as doped single crystal silicon. A thin and electricallyconductive wall structure 20 is around the edge of the mirror 25 with aseveral microns gap between (FIG. 2 c). The upper surface of the wallstructure 20 can be either lower or higher than the lower surface of themirror. The upper surface of the wall structure 20 being lower than thelower surface of the mirror is shown in FIG. 2. The wall structure 20has to be several microns larger than the mirror 25 so that the mirror25 can freely move inwards and outwards relative to the wall structure20 (FIG. 2 c). An electrically conductive supporting material 21 islocated underneath the mirror 25. The thin and electrically conductivewall structure 20 can be either electrically connected to theelectrically conductive support material 21, or electrically isolatedwith the electrically conductive support material through a thin layerof dielectric material 24. In FIG. 2, the thin and electricallyconductive wall structure 20 and electrically conductive supportmaterial 21 are electrically isolated by a thin layer of dielectricmaterial 24.

There are varieties of actuation schemes for the mirror device shown inFIG. 2. One of them is to use the sidewall structure as the actuationelectrode. For example, the electrically conductive support material 21and electrically conductive wall structure 20 may be connected toelectrical ground. If an AC or DC voltage is applied on the mirror 25,there will be electrostatic forces. One electrostatic force is betweenthe mirror 25 and the electrically conductive support material 21, andis a typical parallel plate type electrostatic actuator. The otherelectrostatic force is created between the mirror 25 and the wallstructure 20, and is similar to a staggered vertical comb driveelectrostatic actuator. The vertical comb drive electrostatic actuatorhas many advantages compared with the parallel plate type electrostaticactuator, such as no pull-in effects, higher actuation energy density,etc. These two electrostatic forces will pull the mirror into the wallstructure towards the electrically conductive layer 21. The hinge 26will be deformed. When the electrostatic forces are balanced with themechanical restoring force of the deformed hinge 26, mirror 25 willstabilize at a tilting angle.

The other example of an actuation scheme is to keep the wall structure20 at electrical ground, and apply an actuation voltage to theelectrically conductive support material 21 and the mirror 25. Theelectrostatic force generated between the wall structure 20 and themirror 25 will cause the mirror to tilt at a stable position.

The openings 23 (shown in FIG. 2 b and FIG. 3 a) on the wall structure20 are used to release the squeezed air due to the movement of themirror. The number of openings, the opening size and the location can beoptimized on the wall structure to achieve the desired air damping forthe mirror movement. Suitable air damping is desired for MEMS opticalswitches, MEMS VOA, etc. since it will help the mirror to achieve astable position within a much shorter time using a simpler electricalcontrol scheme.

Referring to FIGS. 3 a and 3 b, the mirror array can be formed using themirror structure shown in FIG. 2 a, FIG. 2 b and FIG. 2 c. If all theelectrically conductive support material 21 is connected to electricalground, and all the wall structures 20 are also connected to electricalground, each mirror can be actuated independently be applying anactuation voltage to it.

The mechanical interference from a squeezed air film is eliminated bythe walls between any two mirrors. The squeezed air film can be releasedor adjusted by the openings 23 and 28 on the wall structure. Since thereare no exposed dielectric materials to the mirrors, no mirror tiltingdrifting will occur.

The interference of the electrical field to tilt the adjacent mirrorwill also be reduced or eliminated. The wall between any two mirrorsprovides an electrical shield between the two mirrors. The gap betweenthe mirror edge and its surrounding wall is much smaller than the gapbetween the two mirrors; therefore, the electrostatic force between themirror and its surrounding wall structure is much higher than theelectrostatic force between the two mirrors. Sometimes, in order toincrease the shielding effects, the top surface of the sidewallstructure can be made at the same height as the mirror surface. FIGS. 2and 3 only show the case where the top surface of the sidewall structureis lower than the mirror surface.

Referring to FIGS. 3 a and 3 b, in order to have a mirror array with ahigh fill factor, the part of the wall structure between two mirrors isvery thin. To improve the strength of this part of wall structure, thesmall recesses 31 on both sides of the mirror 25 are made to give somespace to add a structural post 32 to the wall structure. The recesses 31are preferably small enough that no optical reflective surface area ofmetal coating 19 is sacrificed. Also, the extended portion 30 and 29 ofthe wall structure will be used to improve the strength of the wallstructure.

The possible undesired sideways movement of the mirror can be preventedby using a properly designed hinge shape. FIGS. 4 a and 4 b show asimple short I beam shape hinges 37 and 38 with a mirror connector 40that connects to a mirror supported by fixed anchors 36 and 39 throughthe hinges 37 and 38. The possible undesired sideways movement of themirror can result from different factors. One factor is the alignmenterror in the lithography process, where the gaps between the mirror'stwo long edges and the surrounding wall structure are not equal. Once anelectrical potential is applied between the mirror and its surroundingwall structure, the gap offset results in the unbalanced electrostaticforces on the two long edges of the mirror. As a result, the mirror willnot only tilt around the x-axis and move inwards to its surrounding wallstructure, but also, the unbalanced electrostatic force on the twomirror long edges will force the mirror rotate around the z-axis. Thisrotation around the z-axis is undesirable since it causes electricalshorting once the mirror is contacting the surrounding wall structure.

Referring to FIGS. 22 c and 22 d, a mechanical stop 309 may be used toprevent further mirror rotation around the z-axis and prevent electricalshorting. It is very important to design the hinges so that they havenot only good flexibility to allow desirable rotation around the x-axis,but also the strong stiffness to reduce or eliminate any possibleundesirable rotation around the z-axis. In FIG. 5 a, the mirrorconnector is supported by anchors 44 with double parallel I beams. Thehinge with double parallel I beams design has better rotationflexibility/stiffness control on X and Z axes. The design parameters areA1 (I beam height), A2 (the gap between two I beams), A3 (I beam width)and A4 (I beam length). The rotation stiffness around Z axis will beincreased if A2 (the gap between two I beams) is increased.

The hinge designs can be optimized by using combinational of single Ibeam and single double parallel I beams as shown in FIG. 5 c, single Ibeam and two or more cascaded double parallel I beams as shown in FIG. 5d through Se. For cascaded double parallel I beam structure, the doubleparallel I beam with larger gap A2 is towards to the mirror connector,while the double parallel beams with smaller gap A2 is towards to theanchor. The design parameters A1, A2, A3 and A4 can be varied for eachsingle I beam and double parallel I beams. The hinge designs with otherI beam combination variations are not limited to what is shown anddescribed herein. For example, single I beam with more than threecascaded double parallel I beams, multiple cascaded double parallel Ibeams, part of the hinges consisting of cascaded double parallel I beamsor multiple cascaded double parallel I beams etc.

The V shaped hinge design shown in FIG. 5 f can also be used since ithas good flexibility to allow desirable rotation around X axis, but alsothe strong stiffness to reduce or eliminate any possible undesirablerotation around Z axis. Again, this is not only limited to single Vshape hinge, it also includes the cascaded multiple V shape hinges, asingle I beam with a cascaded V shape hinge as shown in FIG. 5 g, orcombination of V shape hinges and double parallel I beam hinges.

In FIG. 2 and FIG. 3, the mirror can tilt only in one direction. FIG. 6shows a mirror structure which can have the two rotation directionsaround the x-axis defined by the hinge. The mirrors 45 and 46, wallstructures 52 and 53, anchors 47 and 50, hinges 48 and 49 and supportingmaterial 51 are electrically conductive, and may be made of, forexample, typical doped single crystal silicon. The air release openings55 are on the wall structure. The reflective metal costing 56 and 57 areon the top surface of mirror 45 and 46 respectively. The wall structuralsupports and expanded portions 58 are also shown in FIG. 6 a. Theelectrical isolation layer 54 can be, for example, a silicon oxidelayer, which electrically isolates the wall structure 52 and 53 with theelectrically conductive supporting layer 51. The mirror connector has atrenching and refilled region 43, which mechanically connects butelectrically isolated mirror 45 and 46. The anchor 47, hinge 48 andmirror 46 are electrically connected, as are the anchor 50, hinge 49 andmirror 45.

When the electrically conductive supporting layer 51, wall structure 52and 53 are connected to electrical ground, if the anchor 47 is connectedto electrical ground and anchor 50 is connected to an electricalpotential, the resulting electrostatic forces between mirror 45 and wallstructure 52 as well as electrically conductive supporting material 51will pull the mirror 45 towards to the electrically conductive material51 while the mirror 46 moves away from the electrically conductive layer51. If the anchor 50 is connected to electrical ground and the anchor 47is connected to an electrical potential, the resulting electrostaticforces between the mirror 46 and the wall structure 53 as well as theelectrically conductive supporting material 51 will pull the mirror 46towards to the electrically conductive layer 51 while the mirror 45 moveaway from the electrically conductive layer 51. Other actuation schemesalso can be used to tilt the mirrors about an axis in two directions.

A trenching and dielectric material refilling method is used on singlecrystal silicon to form the electrically isolated but mechanicallyconnected regions, examples of which are shown in FIG. 7. The DeepReactive Ion Etching (DRIE) of silicon is often utilized to create ahollow trench that is a couple of microns wide formed on the singlecrystal silicon layer 60. Sometimes, some very narrow silicon structures(about 1 um thick, for example), such as narrow silicon beams or meshingstructures, are kept to connect the intended electrical isolated siliconregions crossing the hollow trenches. These narrow silicon structureswill keep the intended electrically isolated silicon parts togetherduring the whole process. A single trench or multiple parallel trenchesshould be formed in order to have good electrical isolation andmechanical strength. The processed wafer is then sent to the thermaloxidation furnace for wet or dry oxidation, the oxidation will occur onthe sidewalls of the hollow trench. The oxide formed from both sidewallsof the trench will meet each other to close or almost close the trenchafter a certain period of thermal oxidation. Also, if very narrowsilicon structures are used, these tiny silicon structures, such asnarrow silicon beams, will also change into oxide structures afterthermal oxidation. These very narrow silicon oxide structures willmechanically join two electrical isolated silicon regions. Thesubsequent CMP (Chemical and Mechanical Polish) may be performed toremove excess oxide on the both top and bottom side surfaces of thesilicon wafer. There are many other options to do the trench etching anddielectric material refilling. For example, instead of filling thetrench with very thick oxide alone, LPCVD (Low Pressure Chemical VaporDeposition) or PECVD (Plasma Enhanced Chemical Vapor Deposition)polysilicon can be used to fill most of the hollow trenches afterinitial thin thermal oxide growth. The choice of the trench etchingmethod and refilling materials depends on the requirements of themicromirror and associated process cost. FIG. 7 shows three differenttrenching and refilling pattern shapes 61, 62 and 63, each of which hasvery good mechanical strength to connect two electrically isolatedsilicon pieces on their right and left sides.

The mirror structures shown in FIGS. 6 and 8 have a higher mechanicalstability if they are subject to shocking and vibration environmentscompared with the mirror structure shown in FIGS. 2 and 3. In FIG. 8,the anchors 71, hinges 70, and mirrors 67 and 69 are mechanically andelectrically connected. Reflective metal coatings 74 and 75 are on thetops of mirrors 67 and 69, respectively. A layer of electrical isolationmaterial 73 is used to electrically isolate the wall structure 66 andelectrically conductive supporting material 68. Squeezed air releasingopening 72 is on the wall structure 66.

There are two mirrors 67 and 69 on the both side of the hinge 70, asshown in FIG. 8. If the device is subject to the shock force, theresulting inertia force from mirror 67 and 69 will be balanced out; notilting around the hinge, e.g., the x-axis, will be expected. When thewall structure 66 and electrically conductive supporting material 68 areconnected to the electrical ground, and the wall height is intentionallyincreased to the extent that the electrostatic force between wallstructure 66 and mirror 67 is dominant compared with the electrostaticforce between the electrically conductive layer 68 and mirror 75, anyapplied voltage on the mirrors 67 and 69 will cause the minor 67 to movetowards the electrically conductive supporting material 68 and themirror 69 to move away from the electrically conductive layer 68.

The mirror structures shown FIG. 6 and FIG. 8 can be used in a mirrorarray with a high fill factor. FIG. 9 a shows a row of a minor arraymade by arranging the mirror structure shown in FIG. 6 side by side.FIG. 9 b shows a row of a minor array made by arranging the mirrorstructure shown in FIG. 8 side by side. FIG. 9 c shows two rows of amirror array made by arranging the mirror structure shown in FIG. 6 sideby side.

Alternatively, the mirror array configurations with high fill factor mayuse mirror devices, such as those shown in FIG. 6 and FIG. 8, in astaggered or offset array as shown in FIG. 10. Both array configurationsuse only one of two mirror devices shown in FIG. 6 and FIG. 8 to formthe mirror array in FIG. 10 a and FIG. 10 b.

In the mirror device and mirror array structure shown in FIGS. 2, 3, 6,8, 9 and 10, the wall structure and electrically conductive supportingmaterial underneath are shown with an electrical isolation materialbetween. However, depending on the process and material choice, as wellas the desired mirror array structure and performance, the wallstructure (e.g. wall structure 52 in FIG. 6 a) and a layer ofelectrically conductive supporting material (e.g. supporting material 51in FIG. 6 a) can be formed from the same material, for example, a dopedsingle crystal silicon. Therefore, no electrical isolation materiallayer (e.g. 54 in FIG. 6 a) is required between the wall structure andthe electrically conductive supporting material layer. Themicrofabrication process becomes simplified, and actuation controlschemes are also simplified. The corresponding electrically conductivemirror (e.g. mirrors 45 and 46 in FIG. 6 a) will be used as the controladdress electrode for the mirror tilting/rotation around the mirrorhinges (e.g. hinges 48 and 49 in FIG. 6 b). The mirror structure shownin FIG. 6 with no isolation material 54 is shown in FIG. 11.

In order to add more actuation controllability for the mirror tilting,some electrically conductive materials as actuation electrodes are oftenused underneath the mirror, for example, the fixed actuation electrode 4shown in FIG. 1 b. However, the fixed actuation electrode is often madeof a thin layer of metal film. In FIG. 12, the fixed actuationelectrodes 81 and 82 are on top of the dielectric material 83, which ison the support material 84. Due to very thin structure of the electrode81 and 82, the electrical filed resulting from the electrical chargingof the dielectric material 83 in the gap 85 between electrode 81 and 82will affect the tilting of the mirror 80, resulting in spurious movementof the mirror.

One method to shield the undesirable electrical field from the chargeddielectric material and reduce spurious movement is to use the electrodestructure with the high aspect ratio between the electrode thickness andelectrode gap. In FIG. 13 a, the electrode 88 and 90 are very thickcompared with the gap 89 between them. Once the ratio between theelectrode thickness and electrode gap is high enough, e.g. over 10, thenthe electrical field from the charged dielectric material can besignificantly shielded. Therefore, the undesirable interference of themirror tilting can be significantly minimized.

FIG. 13 b shows another implementation, where electrode 91 is verythick, but electrode 93 is relative thinner. Once the gap 92 is smallenough, sufficient shield results can be achieved.

FIG. 13 c shows a structure that is very similar to the mirror devicewith a surrounding electrically conductive wall structure. The electrode94 represents the electrically conductive wall structure, and electrode96 is fixed on the dielectric material 83 and is relatively thin. Butwith a fairly small electrode gap 95, sufficient shield results can beachieved.

The shielding effects of undesirable electrical field from chargeddielectric material can be even improved significantly if the dielectricmaterial in the electrode gap region 100 can etched away so that overhanging electrodes are achieved, as shown in FIGS. 14 a, 14 b and 14 c.

FIGS. 15 a, 15 b and 15 c show the mirror devices with electrodestructures that shield the undesirable electrical fields from chargeddielectric material. In FIG. 15 a, the electrically conductive wall 102has very small gap with the thick electrically conductive electrode 104,which is on top of the dielectric material 103. In FIG. 15 b, the wallstructure 105 is on top of the dielectric material 106, and a relativelythick electrode 107 is inserted in the dielectric material 106. The gapbetween wall structure 105 and electrode 107 is very small, and thedielectric material in the gap region is etched away. In FIG. 15 c, thewall structure 109 and the relatively thin electrode 111 are on top ofthe dielectric material 110, with a small gap between the wall structure109 and the thin electrode 111.

The structures of the electrodes 104, 107 and 111 shown in FIG. 15 maybe fabricated or directly purchased from the commercial vendors. Thesubstrate 114 shown in FIG. 16 a may be Pyrex glass. The thickelectrodes 115 and 116 can be electroplated on the top of the glassplate 114 with a small gap. Furthermore, various different shapes of theelectrodes can be electroplated on the glass plate 114.

The customized silicon inserts in the glass plate can be purchased fromthe commercial supplier. The shape of the thickness and the properties(doping, etc.) of the silicon insets can be specified. In FIG. 16 b, thesilicon inserts 117 and 118 are heavily doped, and can be used asactuation electrodes. The gap between silicon insets 117 and 118 is verysmall. The glass in the area 119 of the gap region is etched away forthe better shielding effects using high aspect ratio electrode gapstructure.

The conventional thin film metallization, lithography and metal filmetching are used to form the electrodes shown in FIG. 16 c. Theprocesses are done on the flat surface of the glass substrate 114, suchthat the gap between electrode 121 and 120 can be made very small.

In order to increase the actuation effects between the mirror and thewall structure for larger actuation forces, some part of the wallstructure of the mirror devices described previously, such as thoseshown in FIGS. 2, 3, 6, 8, 9, 10, 11 and 15, is made into a comb drivestructure. FIG. 17 a and FIG. 17 b show the comb finger structures onthe wall structure part 128 and 132, where the comb finger structuresare also made on the mirror edges corresponding to the comb drivestructures on the wall structure part 128 and 132. The comb fingerstructures on the wall structures and on the mirror edges forms atypical vertical electrostatic comb drive. These vertical comb driveshave advantages of simple structure, easy fabrication and largerresulting electrostatic force.

In FIG. 17 a, the supporting material 126 is made from electricallyconductive material, such as heavily doped single crystal silicon. Thewall structures 128, 129, 133 and 132 are also made from electricallyconductive material, and they are electrically isolated from theelectrically conductive supporting material 126 using dielectricmaterial 127, which can be any suitable dielectric material, such assilicon oxide. The anchors 136 and 137, hinges 138 and 139, mirrors 130and 131 are also made of the electrically conductive material. Thereflective metal coatings are on the top of mirrors 130 and 131. Thewall structures 128 and 132 with comb finger structures are mechanicallyand electrically separated from the remaining wall structures 129 and133. There are a variety of actuation methods of the device shown inFIG. 17 a. One of the actuation methods is described below.

The anchors 136 and 137, hinges 138 and 139, mirrors 130 and 131, wallstructures 129 and 133 are connected to the electrical ground. If thecomb drive of the wall structure 132 is connected to the electricalground and an electrical voltage is applied to the comb drive of thewall structure 128, the resulting electrostatic force between the combstructures of wall structure 128 and mirror 130 will pull the mirror 130to tilt in one direction around the hinges 137 and 139 towards thesupporting material 126. The tilting will also cause the deformation ofthe hinges of 138 and 139. When the mechanical restoring force of thehinges 138 and 139 is balanced by the electrostatic force, the mirrors130 and 131 will remain in the tilted position. If the comb drive of thewall structure 128 is connected to the electrical ground and anelectrical voltage is applied to the comb drive of the wall structure132, the resulting electrostatic force between the comb structures ofwall structure 132 and mirror 131 will pull the mirror 131 to tilt inanother direction around the hinges 137 and 139 towards the supportingmaterial 126. The tilting will also cause the deformation of the hingesof 138 and 139. When the mechanical restoring force of the hinges 138and 139 is balanced by the electrostatic force, the mirrors 130 and 131will stay in another tilted position.

If the supporting material 126 in FIG. 17 a is not an electricallyconductive material, but instead a non-electrically conductive material,such as Pyrex glass, then a lay of electrically conductive materials(shown in FIG. 15) should be applied on the top of the supportingmaterial 126 and under the mirrors to avoid electrical charging effectsof the non conductive supporting material.

FIG. 17 b shows another implementation with vertical comb drive. Thesupporting material 134 is an electrically conductive material. The wallstructures 128, 129, 133 and 132 are made of the electrically conductivematerial, and they are electrically connected to the electricallyconductive supporting material 134. The anchors 136 and 137, hinges 138and 139, mirrors 130 and 131 are also made of the electricallyconductive materials. The reflective metal coatings are on the top ofmirror 130 and 131. The trenching and refilling region 135 is on themirror connector between the hinges 138 and 139. The region 135mechanically connects but electrically isolates the separated mirrorconnector, so the mirror 130 and mirror 131 are mechanically connectedbut electrically isolated. There are a variety of actuation methods ofthe device shown in FIG. 17 b. One of the methods is described below.

If the wall structures 128, 129, 132 and 133, supporting material 134,anchor 137, hinge 138 and mirror 131 are all connected to the electricalground, and an electrical voltage is applied to the anchor 136, hinge139 and mirror 130, the resulting electrostatic force between the combfinger structures of wall structure 128 and mirror 130 will pull themirror 130 to tilt in one direction around the hinges 137 and 139towards the supporting material 134. The tilting will also cause thedeformation of the hinges of 138 and 139. When the mechanical restoringforce of the hinges 138 and 139 is balanced by the electrostatic force,the mirrors 130 and 131 will stay in a tilted position. If the wallstructures 128, 129, 132 and 133, supporting material 134, anchor 136,hinge 139 and mirror 130 are all connected to the electrical ground, andan electrical voltage is applied to the anchor 137, hinge 138 and mirror131, the resulting electrostatic force between the comb fingerstructures of wall structure 132 and mirror 131 will pull the mirror 131to tilt in another direction around the hinges 137 and 139 towards thesupporting material 134. The tilting will also cause the deformation ofthe hinges of 138 and 139. When the mechanical restoring force of thehinges 138 and 139 is balanced by the electrostatic force, the mirrors130 and 131 will stay in another tilted position.

If the supporting material 134 in FIG. 17 b is not electricallyconductive material, such as doped silicon, but a non-electricallyconductive material such as Pyrex glass, then a lay of electricallyconductive materials (shown in FIG. 15) should be applied on the top ofthe supporting material 134 and under the mirrors to avoid electricalcharging effects of the non conductive supporting material.

Sometimes, it is important to have a higher fill factor for the mirrorarray. In this case, the whole or part of wall structure 129 and 133between two adjacent mirrors can be removed to reduce the gap betweenany two adjacent mirrors. FIG. 17 c and FIG. 17 d show the mirrordevices shown in FIGS. 17 a and 17 b without wall structure 129 and 133between two mirrors respectively. Vertical comb dive electrostaticactuators in 128 and 132 in FIGS. 17 a, 17 b, 17 c and 17 d are thedominant actuators.

There are varieties of actuation methods of the device shown in FIG. 17c. One of the actuation methods is described below.

The anchors 136 and 137, hinges 138 and 139, mirrors 130 and 131 areconnected to the electrical ground. If the comb drive of the wallstructure 132 is connected to the electrical ground, and an electricalvoltage is applied to the comb drive of the wall structure 128, theresulting electrostatic force between the comb finger structures of wallstructure 128 and mirror 130 will tilt the mirror 130 in one directionaround the hinges 137 and 139 towards the supporting material 126. Thetilting will also cause the deformation of the hinges of 138 and 139.When the mechanical restoring force of the hinges 138 and 139 isbalanced by the electrostatic force, the mirrors 130 and 131 will stayin a tilted position. If the comb drive of the wall structure 128 isconnected to the electrical ground, and an electrical voltage is appliedto the comb drive of the wall structure 132, the resulting electrostaticforce between the comb finger structures of wall structure 132 andmirror 131 will pull the mirror 131 to tilt in another direction aroundthe hinges 137 and 139 towards the supporting material 126. The tiltingwill also cause the deformation of the hinges of 138 and 139. When themechanical restoring force of the hinges 138 and 139 is balanced by theelectrostatic force, the mirrors 130 and 131 will stay in another tiltedposition.

If the supporting material 126 in FIG. 17 c is not an electricallyconductive material such as doped silicon, but an electricalnon-conductive material such as Pyrex glass, then a lay of electricallyconductive materials (shown in FIG. 15) should be applied on the top ofthe supporting material 126 and under the mirrors to avoid electricalcharging effects of the non conductive supporting material.

The mirror devices with more actuation flexibilities and lower actuationvoltage are shown in FIGS. 18 a, 18 b and 18 c. These mirror deviceshave three actuation components, general wall structures between twomirrors, and comb structures on the part of wall structure and bottomelectrodes on the supporting material. The three actuation componentscan be used at the same time, can be used with any two components, orcan be used independently. The trenching and refilling regions are notshown on the mirror devices in FIGS. 18 a, 18 b and 18 c, but can beimplemented in all three mirror devices. The supporting materials 141 inFIGS. 18 a, 18 b and 18 c can be a dielectric material, such as Pyrexglass. The electrodes 142, 143 and 144 form high aspect ratio gaps withthe wall structures. These high aspect ratio gaps have very goodelectrical shielding effects to the electrical field of the chargeddielectric materials which are exposed to the mirrors. The etching awayor removal of the dielectric material within the gaps (shown in FIGS. 14a, 14 b and 14 c) can further increase the shielding effects and reducethe undesired charging effects.

Sometimes, it is important to have a higher fill factor for the mirrorarray. In this case, all or part of general wall structure between twoadjacent mirrors should be removed to reduce the gap between any twoadjacent mirrors. FIGS. 18 d, 18 e and 18 f shows the mirror devicesshown in FIGS. 18 a, 18 b and 18 c without general wall structurebetween two adjacent mirrors respectively.

The comb drive structures on the mirror edges or on the part of wallstructures form vertical electrostatic comb drives. There are many combfinger configurations of such vertical comb drive actuators. FIG. 19shows four different comb drive finger configurations. The comb fingersare perpendicular to the tilting x-axis in FIG. 19 a, and the combfingers are parallel to the tilting x-axis in FIG. 19 b. The combfingers may also have a small angle with the tilting axis X as shown inFIG. 19 d. The comb finger configuration has many variations besides theabove-mentioned three comb finger configurations. For example, the combfingers can be a curved beam, multiple curved beams or multiple straightbeams, as long as the effective comb finger has very small angle withthe tilting x-axis. In addition, combinations of the above three combfinger configurations can also be used. FIG. 19 c shows the combinationof comb finger configurations in FIGS. 19 a and 19 b.

For the purposes of having different tilting angle and structurestrength as well as convenience to form the mirror array with high fillfactor, the electrostatic vertical comb drives are arranged at suchlocations shown in FIGS. 20 a and 20 b. There are two vertical combdrives in FIG. 20 a. One is close to the tilting hinges, and the otheris spaced from the tilting hinges. When the vertical comb drive close tothe hinge is used, a larger mirror tilt can be expected. The twovertical comb drives are all close to the tilting hinges shown in FIG.20 b, therefore when any one of the actuators works, a larger mirrortilt can be expected.

All the mirror structure shown in FIGS. 17 a-18 f, 20 a and 20 b can beused to form the high fill factor mirror arrays shown in FIG. 9 and FIG.10.

The mirror device can also be designed in such way that the mirror 146in FIG. 21 used for optical switching, etc. is on one side of thehinges, while the shorter and wider part 147 in FIG. 21 is on the otherside of hinged as a weight balance mirror. These mirror structures havemany advantages, such as forming a smaller footprint mirror array. Thepurpose of part 147 is to keep the mirror 146 in the stable positions ifthe whole mirror device is subjected to the shock and vibrationenvironments. Since part 147 is not used as an optical mirror, it can bedesigned using shorter and wider shapes. FIG. 21 a shows a mirror devicewith a retrenching and refilling region on the mirror connector. FIG. 21b shows a mirror device that has comb fingers on the mirror edges and aweight balanced mirror edge. If the actuation electrodes underneath themirror 146 and weight balanced mirror 147 are required, the high aspectratio electrode gap with electrical field shielding effect should beused in all these mirror devices.

In order to have a mirror array with an even higher fill factor, thewhole or part of the wall structure between two adjacent mirrors inFIGS. 21 a and 21 b can be removed.

FIG. 21 c shows the comb finger structures 342 connected to the mirrors341 through extension beams 340. The purpose of extension beam is toform higher fill factor mirror arrays when the mirror pitch is gettingsmaller while wider space is required for these comb finger structure.

The high fill factor mirror array shown in FIG. 22 a can be formed usingmirror device shown in FIG. 21 a, while the mirror array in FIG. 22 bcan be formed using mirror device shown in FIG. 21 b. Both mirror arrayshave smaller foot prints. The mirror array shown in FIGS. 22 a and 22 bcan be also formed with the mirror devices shown in FIGS. 21 a and 21 bwithout a wall structure between two adjacent mirrors.

The high fill factor mirror array shown in FIG. 22 c can be formed usingthe mirror device shown in FIG. 21 c. FIG. 22 d shows a simplifiedversion of the mirror array shown in FIG. 22 c in order to reduce thedie size and cost. The wall structure between two adjacent mirrors canbe partly or totally removed depending on the requirements of mirrorfilling factor. In FIGS. 22 d and 22 c, the hinges 312 and 313 ofadjacent mirrors 305 and 306 are staggered in order to accommodate thewider hinge for the mirror array with smaller mirror pitch, where themirror pitch is the distance between two adjacent mirror centers. Thecomb finger structures 301 and 303 are connected to the mirrors 305 and306 using extension beams 308 and 307, respectively. The fixed combfinger structures 300 and 302 are separated by DRIE trenches andconductive electrical shielding traces 310. The mechanical stop 309 isused to prevent the excess undesirable side way movement of the mirror.

The mirror device shown in FIG. 23 has the ability to tilt around both Xand Y axes. In FIG. 23 a, the anchor 150 is fixed and within a gimbalhinge structure formed by the combination of single I beam and cascadeddouble I beams. The vertical comb drive 148 is used to tilt the mirrorsin one direction around axis X, while the vertical comb drive 154 isused to tilt the mirrors in another direction around axis X. Thevertical comb drive 149 is used to tilt the mirrors in one directionaround axis Y.

In FIG. 23 b, the actuation electrodes on the supporting material areused to increase the actuation ability. The electrode 151 can work withvertical comb drive 152 to pull the mirror 153 toward itself. For bothdevices shown in FIGS. 23 a and 23 b, again the general wall structurebetween two adjacent mirrors can be partly or totally removed toincrease the mirror fill factor.

The high fill factor mirror array shown in FIG. 24 a can be formed usingmirror device shown in FIG. 23 b, while the mirror array in FIG. 24 bcan be formed using mirror device shown in FIG. 23 a. Both mirrordevices can be also configured to form the high fill factor mirror arrayshown in FIG. 25. Again, the mirror array shown in FIG. 24 and FIG. 25can be formed by the mirror devices shown in FIG. 15, 17, 18, 20, 21 or23, while the general wall structures between two adjacent mirrors ofthese mirror devices may be partly and totally removed to increase themirror fill factor. Also, these mirror device may use extension beams toconnect the comb drive fingers to the mirror for the mirror array withsmaller mirror pitches, the mirror pitch is the distance between twoadjacent mirror centers.

Many microfabrication methods and materials can be used to make themicromirrors and micromirror arrays described herein. Twomicrofabrication process flows and corresponding materials will bedescribed in the following paragraphs. Only major process steps aredescribed. The process methods and materials are not limited to what isdescribed in these two microfabrication methods.

The first process flow shown in FIG. 26 gives a very simple processflow. The purpose of this process flow is for making a lower budget 1X2WSS micromirror array, VOA, optical switch etc. A SOI (Silicon onInsulator) wafer (shown in FIG. 26 a) is used as the starting material.It has thin oxide layer 155, silicon device layer 158, which is heavilydoped to increase the electrical conductivity, buried oxide layer 156and handle silicon wafer 157. The trenching and refilling regions canalso be made in the silicon device layer 158. The trenching andrefilling regions are not shown in the process flow. Such SOI wafer canbe custom made by silicon wafer vendors.

The first lithography and subsequent oxide RIE (Reactive Ion Etching)are used to pattern the oxide layer 155 (FIG. 26 b). The patterned oxidelayer serves as the electrical isolation layer between silicon devicelayer and supporting silicon 159. The cavities are created using siliconDRIE on silicon device layer 158 (FIG. 26 c). A heavily doped siliconwafer 159 used as supporting material and is bonded with the SOI waferusing wafer bonding (FIG. 26 d) such as fusion bonding, eutectic bondingetc. The handle silicon wafer 157 of the SOI wafer is removed usingchemical wet etching or silicon CMP (Chemical and Mechanical Polishing)(FIG. 26 e). The second lithography and subsequent oxide RIE (ReactiveIon Etching) are used to pattern the buried oxide layer 156 (FIG. 26 f).The patterned oxide layer serves as the subsequent silicon DRIE etchingmask. The thin film metallization is applied to the wafer using E-beamevaporation or sputtering. The third lithography and metal etching orliftoff are utilized to create the reflective metal coating (160) on themirror, the metal bonding pads and electrical interconnection traces(FIG. 26 g). A layer of photoresist 161 is coated and patterned to coverall the metal patterns in order to protect them during subsequentsilicon DRIE and oxide RIE (FIG. 26 h). A silicon DRIE is used torelease the mirrors and create the hinges etc. (FIG. 26 i). An oxide RIEremoves the remaining buried oxide used as DRIB etching mask (FIG. 26j). The last photoresist ashing process clears away the protectionphotoresist layer 161.

There are many process variations to the above process method. Forexample, a regular doped single crystal silicon wafer may be used as thestarting and supporting material. After thermal oxidation and cavityformation using silicon DRIE, it is bonded with second doped singlecrystal silicon wafer, which will be used to form the mirrors and hingesetc. There may be an electrical isolation oxide between the two wafers.After thinning of the second silicon wafer to the right thickness usingCMP, the similar process steps (FIGS. 26 e-26 k) can be used tofabricate the mirror and micromirror array.

Another variation of the first process flow is used to maintain theaccurate mirror, hinge thickness, cavity depth as well as comb fingerheights. In this process variation, a SOI wafer is used as a startingmaterial, its device silicon thickness is same as the cavity depth. Atthe process step shown in FIG. 26 c, the DRIE etching will etch thedevice silicon all the way to the buried oxide 156 to form the lowercomb drive fingers and cavities underneath the mirrors, such that allthe exposed buried oxide 156 within the opened comb finger cavities andcavities underneath the mirrors will be etched away. So the starting SOIwafer becomes a supporting wafer. The wafer 159 in this processvariation could be a second SOI wafer, whose doped device silicon layerhas accurate desired thickness required for the mirrors, hinges andupper comb fingers. After wafer bonding and handle wafer removal of thesecond SOI wafer, the process steps shown in FIGS. 26 e to 26 k will beperformed on the device layer of the second SOI wafer.

The first process flow only needs three or four masks to fabricate themircromirror or micromirror array devices. The production yield could bevery high and the cost could be very lower.

The second process flow provides a more flexible design of micromirrorand micromirror array.

A double SOI (Silicon on Insulator) wafer (shown in FIG. 27 a) is usedas the starting material. It has a first silicon device layer 170, whichis heavily doped to increase the electrical conductivity, a first buriedoxide layer 171, a second silicon device layer 172, which is heavilydoped to increase the electrical conductivity, a second buried oxidelayer 173, and a handle silicon wafer 174. The trenching and refillingregions can also be made in the silicon device layer 172. The trenchingand refilling regions are not shown in the process flow. Such double SOIwafer can be custom made by silicon wafer vendors.

The first lithography and subsequent silicon DRIE (Reactive Ion Etching)are used to create the cavities underneath the mirrors and lower combfingers in the silicon layer of 170 (FIG. 27 b). The first buried oxidein the bottom of the cavities is etched away either by oxide RIE oroxide wet etching.

A Pyrex glass wafer 175 with metal patterns (176) on its one side willbe anodic bonded with the double SOI wafer (FIGS. 27 c and 27 d). Themetal pattern and structures can be any one of configurations shown inFIGS. 16 a, 16 b and 16 c. For the purpose of simple description, onlythin film metal pattern is shown in the process flow.

After anodic bonding of double SOI wafer and the Pyrex wafer (FIG. 27d), the handle silicon wafer 174 of the double SOI wafer is removedusing chemical wet etching or silicon CMP (Chemical and MechanicalPolishing) (FIG. 27 e). Lithography and subsequent oxide RIE (ReactiveIon Etching) are used to pattern the buried oxide layer 173 (FIG. 27 f.The patterned oxide layer serves as the subsequent silicon DRIE etchingmask. The thin film metallization is applied to the wafer using E-beamevaporation or sputtering. Another lithography and metal etching orliftoff step is used to create the reflective metal coating (177) on thesilicon mirror, and the metal bonding pads and electricalinterconnection traces (FIG. 27 g). A layer of photoresist 178 is coatedand patterned to cover all the metal patterns and other silicon surfacesin order to protect them during subsequent silicon DRIE and oxide RIE(FIG. 27 h). A silicon DRIE is used to release the mirrors, create uppercomb fingers, the hinges, etc. (FIG. 27 i). An oxide RIB removes theremaining buried oxide used as DRIB etching mask (FIG. 27 j). The lastphotoresist ashing process clears away the protection photoresist layer178.

Using the fabrication flow described in FIG. 27 a-FIG. 27 k, if thesilicon layer 172 and buried oxide layer 171 (FIG. 27 a) between theadjacent mirrors are etched away, then we Will have similar micromirrorarray structures (shown in FIG. 18). In order to further reduce themechanical crosstalk and electrical interference between adjacentmicromirrors, a thin mechanical wall between adjacent two micromirrorsas high as up to the mirror surface may be used. FIG. 28 shows themicromirror array devices with thin wall structure as high as up to themirror surface between adjacent two micromirrors. Ideally, the thin wallmaterial should be electrically conductive. Using the fabrication flowdescribed in FIG. 26 a-FIG. 26 k and FIG. 27 a-FIG. 27 k, we will havesimilar micromirror array structures (shown in FIG. 28) with a thinelectrically conductive silicon wall (190 in FIG. 28) between any twoadjacent mirrors. In order to prevent or reduce the optical reflectionfrom the top surface of the thin wall, the wall structure can be verythin, or its top surface has many some etched thin and fine structuressuch as lines, meshes etc. for reducing the optical reflection.

The same design and microfabrication methods may be used to make thesingle larger titling mirror devices which have applications for opticalswitches, optical attenuators, etc. FIGS. 29 a and 29 b show examples oflarger tilting mirrors. In FIGS. 29 a and 29 b, the mirror 199 has thereflective coating 198 on its top surface. The mirror 199 is connectedto anchor 201 using hinges with combinations of single I beam andcascade double I beams. The comb fingers are on the mirror edge, whichis far from the mirror tilting axis X. In other implementations, theoutside mirror edge without comb fingers can be used as electrostaticactuation components working with the wall. Mirror 199, hinges 200,anchor 201 and comb fingers are all made of electrically conductivematerial such as doped single crystal silicon. The wall structures 202and 191 are made of electrically conductive material such as dopedsingle crystal silicon. They are mechanically and electrically separatedby etching cut 192.

In FIG. 29 a, the supporting material 194 is made of electricallyconductive material such as doped single crystal silicon. It iselectrically isolated from the wall structure 202 and 191 usingdielectric material 203 which can be silicon dioxide. There are manyactuation methods which can be utilized. When the mirror 199 with itscomb fingers and the comb fingers of the wall structure 191 areconnected to the electrical ground, if the comb fingers of the wallstructure 202 is connected to electrical potential, the mirror willrotate around the x-axis in one direction under the electrostatic forcebetween the mirror comb fingers and the comb fingers of the wallstructure 202. When the mirror 199 with its comb fingers and the combfingers of the wall structure 202 are connected to the electricalground, if the wall structure 191 is connected to electrical potential,the mirror will rotate around the x-axis in the other direction underthe electrostatic force between the its comb fingers and the combfingers of the wall structure 191.

In FIG. 29 b, the supporting material 195 is made of dielectric materialsuch as Pyrex glass. It has two electrically isolated metal electrodes196 and 197 on its top surface. There are many actuation methods thatmay be used. When the mirror 199 with its comb fingers and the combfingers of the wall structure 191 and the metal electrode 197 areconnected to the electrical ground, if the comb fingers of the wallstructure 202 and metal electrode 196 are connected to an electricalpotential, the mirror will rotate around the x-axis in one directionunder the electrostatic force between the mirror comb fingers and thecomb fingers of the wall structure 202, as well as the electrostaticforce between the mirror and the electrode 196. When the mirror 199 withits comb fingers and the comb fingers of the wall structure 202 areconnected to the electrical ground, if the comb fingers of the wallstructure 191 and metal electrode 197 are connected to an electricalpotential, the mirror will rotate around the x-axis in the otherdirection under the electrostatic force between the mirror comb fingersand the comb fingers of the wall structure 191, as well as theelectrostatic force between the mirror and the electrode 197.

FIG. 29 c shows another single larger titling mirror; FIG. 29 c showsthe two comb finger pairs 320 and 321, which are consisting of the combfingers on the mirror edges and comb fingers on the wall structures. Themirror will be actuated to tilt around the hinge 322 when any one ofcomb finger pairs 320 or 321 is energized. A simple straight I beamhinge is used in the device shown in the FIG. 29 c, but other hingeshapes, such as a V-shaped hinge, double parallel I beam hinge,composite hinges, etc. can also be used. This simple mirror device hasadvantage of larger tilting angle at lower actuation voltage.

FIG. 29 d shows another single larger mirror which can be actuated in adirection perpendicular to the mirror surface. The vertical comb drives320 and 321 formed by the comb finger on the mirror edges and wallstructure surrounds the mirror. The structural design and locations ofthe hinges 323 supports the mirror to ensure that the mirror will bemoved in the direction perpendicular to its surface to prevent mirrortilting. This type mirror device can be very useful in tunable filters,etc.

In some applications, such as an optical protective switch, it isbeneficial to maintain the desired position of the micromirror even whenthe electrical actuation power is off. In the past, many efforts weretried to achieve the micromirror with latching mechanisms, but there areobstacles that prevented this, such as including stiction effects.

A MEMS thermal actuator has certain advantages, including its simplestructure, easy fabrication and a larger actuation force output, as theactuation force from the thermal actuator can overcome the stictionforce.

The latching structure and the thermal actuator described herein can beused with any MEMS micromirror and micromirror array device. In order toshow the latching structure implementation, FIGS. 30 a and 30 b show anexample with a single larger tilting micromirror with latching structureand a thermal actuator. The same latching and thermal actuator can beimplemented in any mirror such as the mirror used in the mirror array,mirrors shown in FIGS. 29 a, 29 b, 29 c and 29 d to latch these mirrorinto desired positions.

Referring to FIGS. 30 a and 30 b, the thermal actuator is made from asingle crystal silicon. It has two fixed anchors 226, thin siliconexpansion beams 220 and a beam connector 221. The beam 220 can be asingle beam, double beam, multiple beams or any combinations of thesebeam structures. The cross section of the beam 220 shown in FIG. 30 is adouble I beam shape, although it will be understood that other crosssection shapes can also be used. The beam 220 and beam connector 221between fixed anchors 226 are above an etched cavity in order to reducethermal loss and improve actuation efficiency. The shape of beam 220 andbeam connector 221 is preferably V shaped, arc shaped, or other shapesthat will thermally expand in a predictable direction, and that havelarge amounts of thermal deformation in that direction. These shapeswill help to pre-set the actuation direction of the thermal actuator.When the electrical voltage is applied on the two fixed anchors 226, theelectrical current will flow through the silicon expansion beam 220, thetemperature of the expansion beams 220 will increase due to the Jouleheating effect, resulting in the length of the expansion beams alsoincreasing. Since the two fixed anchors limit the outwards movement ofthe beams 220 along their length's direction, the beams 220 could onlymove sideways in the direction determined by its shape. In the case of aV-shaped or arc shaped relationship, the beams will deform in theactuation direction shown in FIG. 31 b.

As shown, a long beam 223 is connected to the mirror 227 and located onthe outside of the mirror 227. A latching structure 225 is formed closeto the tip of the beam 223. The shape of the latching structure 255 canbe any shape, such as arrow shape, that is capable of locking againstanother latching structure or surface. The part of sidewall of latchingstructure in the example is smooth cylinder surface. Other surfaceshapes with smooth tilting surfaces can also be used, but the preferreddesigns offer easy latching process.

As shown, one or two beams 222 are connected to the beam connector 221,such that they have latching structures 224 on their tips. Two beams 222are shown in FIG. 30. The shape of beam 222 can be other shape such asserpentine shape to have more flexibility for easy of engaging into thelatching position. The locking structure 224 can also be located withinthe connector 221 without using beam 222. The part of sidewalls oflatching structure (224) in the example is smooth cylinder surface.Other surface shapes, such as smooth tilting surfaces, can also be usedas long as they can offer easy latching process.

When the thermal actuator is actuated, the beams 222 will be pushedforwards until the cylinder surface of their latching structure 224contacts to the cylinder surface of their latching structure 225. Sincethe thermal actuator can generate very strong actuation forces, theactuation force will be sufficiently strong that it can force thedeformation of the beams 222 and overcome the stiction force betweenlatching structures 224 and 225. The latching structures 224 thereforewill move forwards and slide along the cylinder sidewall surface of thelatching structure 225. Once the latching structures 224 pass over thelatching structure 225, the beams 222 will return from their deformedshapes shown in FIG. 31 a.

If the electrical power for the thermal actuator is off, the mechanicalrestoring force of the beams 220 will pull the beams 222 and theirlatching structures 224 backwards until the latching structure 224contacts with the latching structure 225, as shown in FIGS. 32 a and 32b. At that moment, the mechanical restoring force of the silicon beams220 will lock the mirror in certain position through mechanical contactforce and friction force between the locking structures 224 and 225.

If it is desired to change the tilting and latched position of themirror, electrical power can be applied to the thermal actuator untilthe thermal expansion of the silicon beams 220 overcomes the stictionforce between latching structures 224 and 225, as well as mechanicalrestoring force of the silicon beams 220, and the beams 222 and thelocking structures 224 will move to create a separation gap 228 as shownin FIG. 31 b between latching structures 224 and 225. The mirror thencan freely tilt to the desired position with the help of itselectrostatic actuators. Once the desired mirror position isestablished, the electrical power of the thermal actuation is turnedoff, the mechanical restoring force of the silicon beams 220 will lockthe mirror at the desired mirror tilting position as shown in FIGS. 32 aand 32 b, even after the electrical power for the mirror tiltingactuator has been turned off.

In order to prevent electrical interference between the mirror tiltingactuator and thermal actuator, referring to FIGS. 33 a and 33 b, atrenching and dielectric material refilling region can used toelectrically isolate both actuators. The dovetail shape trenching anddielectric material refilling regions 238 and 234 are on the beamconnector 221 and locking beam 223 respectively. The regions 237 and 239are electrical isolated, as are regions 236 and 235. The materials andfabrication methods of the trenching and dielectric material refillingregion have been described previously. The trenching and refillingregion shape can also be any shape, examples of which are shown in FIG.7.

Since the actuation force of the thermal actuator is too strong forcertain designs, sometimes the strong actuation and mechanical restoringforce from the thermal actuator can push or pull the mirror away fromits desired center balanced position. Therefore, a pair of the thermalactuators may be required to balance the adverse effects of the thermalactuator to the mirror and its hinges. In the FIG. 32 a, another thermalactuator with its latching structure could be built in the position ofthe 230. In some circumstances, more thermal actuators with theirlocking structures may be provided around the mirror to optimize thebalanced latching performance.

It will be understood that many types of MEMS thermal actuators may beused. An example of an alternative thermal actuator is shown in FIG. 34.The thermal actuator is made of an electrically conductive material,such as doped single crystal silicon, and has two fixed anchors 353 and354, thin expansion beam 350, thick beam 351, and latching structure355. The latching structure 356 is connected to the mirror through thebeam 357. When the electrical voltage is applied on the two fixedanchors 353 and 354, the electrical current will flow through the thinand thick beams 350 and 351, and the temperature of both beams will risedue to the Joule heating effects. The temperature of the thin beams 350will be higher than that of the thick beam 351 since the thin beam has alarger electrical resistance, therefore the thermal expansion of thethin beam is larger than that of the thick beam. Also, because the twofixed anchors limit the outward movement of both beams along theirlength's direction, the beams 350 could only push the latching structure355 towards the latching structure 356 into the final engaged latchingposition.

All the thermal actuators described herein can be used in the tiltingmirrors piston movement mirrors, mirror arrays, etc. describedpreviously.

The tilting micromirror and piston movement micromirror with lockingstructures and thermal actuator (FIGS. 29 a, 29 b, 29 c, 29 d, 30 a, 30b, 31 a, 31 b, 32 a, 32 b, 33 a, 33 b and 34) can be fabricated usingthe process flow described herein such as the process shown in FIGS. 26a-26 k, 27 a-27 k.

In this patent document, the word “comprising” is used in itsnon-limiting sense to mean that items following the word are included,but items not specifically mentioned are not excluded. A reference to anelement by the indefinite article “a” does not exclude the possibilitythat more than one of the element is present, unless the context clearlyrequires that there be one and only one of the elements.

While the device described above is susceptible of embodiments in manydifferent forms, the drawings and description give details of preferredembodiments with the understanding that the present disclosure is to beconsidered as an exemplification of the principles of the device and isnot intended to limit the broad aspects of the device to the embodimentsillustrated. The figures are not necessarily drawn to scale and relativesizes of various elements in the structures may be different than in anactual device.

The following claims are to be understood to include what isspecifically illustrated and described above, what is conceptuallyequivalent, and what can be obviously substituted. Those skilled in theart will appreciate that various adaptations and modifications of thedescribed embodiments can be configured without departing from the scopeof the claims. The illustrated embodiments have been set forth only asexamples and should not be taken as limiting the invention. It is to beunderstood that, within the scope of the following claims, the inventionmay be practiced other than as specifically illustrated and described.

What is claimed is:
 1. A MEMS micromirror structure comprising: amovable structure mounted on a support structure by a resilientstructure, and at least a portion of the movable structure comprising amicromirror; an electrostatic actuator for pivotally driving the movablestructure, the electrostatic actuator comprising a first part carried bythe support structure, and a second part carried by the movablestructure; the resilient structure comprising a first portion and asecond portion that is symmetrical to the first portion, each of thefirst portion and the second portion comprising an I beam connected to acomposite structure.
 2. The MEMS micromirror structure of claim 1,wherein the composite structure is one of one or more dual I beamstructures, one or more V-shaped structures, and combinations thereof.3. The MEMS micromirror structure of claim 1, wherein the electrostaticactuator is a vertical comb drive.
 4. The MEMS micromirror structure ofclaim 3, wherein one of the first part of the vertical comb drive andthe second part of the vertical comb drive comprises fingers that areenclosed within an outer perimeter of the other of the first part or thesecond part.
 5. The MEMS micromirror of claim 4, wherein the fingers arecarried by a carrier portion that is perpendicular to the pivot axis,the carrier portion is connected to an external portion that is outsidethe outer perimeter of the other of the first part and the second part.6. The MEMS micromirror of claim 4, wherein the fingers are arrangedparallel to the pivot axis.
 7. The MEMS micromirror structure of claim4, wherein the fingers are angled relative to the pivot axis.
 8. TheMEMS micromirror structure of claim 1, further comprising a cavitybetween the movable structure and the support structure.
 9. The MEMSmicromirror structure of claim 1, further comprising a latch mounted tothe support structure by a movable portion that moves in response to anapplied voltage between a latching position and a release position asthe applied voltage is varied, wherein, in the latching position, thelatch secures the movable structure in a desired position.
 10. The MEMSmicromirror structure of claim 9, wherein the movable portion is athermal arched beam actuator.
 11. The MEMS micromirror structure ofclaim 9, wherein the movable portion comprises first and second parallelthermal connectors that expand at different rates in response to theapplied voltage.
 12. An array of MEMS micromirrors, and each micromirrorcomprising: a movable structure mounted on a support structure by aresilient structure, and at least a portion of the movable structurecomprising a micromirror; an electrostatic actuator for pivotallydriving the movable structure, and the electrostatic actuator comprisinga first part carried by the support structure, and a second part carriedby the movable structure; and the resilient structure comprising a firstportion and a second portion that is symmetrical to the first portion,and each of the first portion and the second portion comprising an Ibeam connected to a composite structure.